We have investigated the mechanism by which the DNA of bacteriophage lambda integrates into the chromosome of its Escherichia coli host. We studied the role of DNA homology between the recombining sequences and ruled out two plausible alternatives for this requirement. We also determined that the two recombination partners interact with recombination proteins very differently. The bacterial partner obtains its recombinase only by collision with a nucleoprotein assembly formed on the viral partner. The genes for a second recombination protein have been placed on an overexpression vector and thus enabled the synthesis of large quantities of active protein.